BACKGROUND: Although adults with developmental disabilities are at high risk for obesity and its sequelae, few community-based lifestyle interventions targeting those with developmental disabilities exist. DESIGN: The study was a single group, community-based demonstration project with pre-post test evaluation conducted from December 2005 to June 2006. SETTING/PARTICIPANTS: Eligible participants were 431 community-dwelling adults with developmental disabilities, aged 18-65 years, who were overweight/obese (BMI > or =25) with another risk factor for diabetes or metabolic syndrome or who had a diagnosis of diabetes, and received services from a community agency. Eighty-five signed up (20% of those eligible), 68 participated in an initial class, and 44 completed the program (35% attrition rate). INTERVENTION: The Healthy Lifestyle Change Program (HLCP) is a community-based health intervention developed and implemented using community-based participatory research methods by members of the developmental disabilities community, in collaboration with academic researchers. The HLCP was a 7-month, twice-weekly education and exercise program to increase knowledge, skills, and self-efficacy regarding health, nutrition, and fitness among adults with developmental disabilities. Peer mentors served as participant leaders and primary motivators. MEASURES: Changes in weight, BMI, abdominal girth, access to care, and self-reported nutrition, physical activity, and life satisfaction were each measured. RESULTS: Two thirds of participants maintained or lost weight, with a mean weight loss of 2.6 pounds and a median weight loss of 7 lbs (range: 2-24 lbs). Average BMI decreased by 0.5 kg/m(2) (p=0.04). Abdominal girth decreased in 74% of participants (mean= -1.9 inches). Sixty-one percent of participants reported increased physical activity. Mean exercise frequency increased from 3.2 times to 3.9 times per week (p=0.01). Mean exercise duration increased from 133 minutes to 206.4 minutes per week (p=0.02). Significant improvements in nutritional habits and self-efficacy were reported. Over half (59%) of participants showed improvements in life satisfaction. Participants received 206 referrals for needed medical care. The HLCP and its dissemination increased participants' and peer mentors' ability to act as community advocates and partners in research. CONCLUSIONS: The HLCP resulted in improved lifestyles, weight loss success, and increased community capacity, indicating that a community-based program with significant participation of those with developmental disabilities is feasible. This program should be expanded and evaluated with larger populations with developmental disabilities.